FIG. 1 shows a diagrammatical structure of a modulator using a quadrature phase shift keying (QPSK) method known in the art. A transmission signal supplied as base band data is decomposed into single bits i.sub.k and q.sub.k alternately by a distributor 2, and is inputted to balanced modulators 8 and 10 through roll-off filters 4 and 6, respectively. A binary phase shift keying (BPSK) signal is obtained at each output of the balanced modulators 8 and 10. The phase of the carrier wave generated by a carrier wave generator 12 is shifted by 90 degrees by a phase shifter 14, and hence the phases of the two carrier waves differ by 90 degrees. Therefore, a quadrature PSK signal is obtained by composing vectors of the two BPSK signals by an adder 16, and then by removing unnecessary frequency components by a band-pass filter 18.
The modulated signal by QPSK is expressed by the following equation. ##EQU1##
In this case, a modulated signal by BPSK is obtained at the output of either the balanced modulator 8 or 10 shown in FIG. 1, and is expressed by the following equation. EQU S(t)=(i.sub.k cos.omega..sub.c t (1A)
FIG. 2 shows phase changes in QPSK. Let us consider the case where the phase of the signal changes from the signal point P.sub.1 to P.sub.2 (a phase rotation amount is 180 degrees), or from the signal point P.sub.1 to P.sub.4 (a phase rotation amount is 90 degrees). In an ideal case where these phase changes occur in zero time, the phase locus passes along the I-axis or the Q-axis, and the envelope of the QPSK signal is maintained constant. Actual phase changes, however, take finite times, and the phase loci are apart from the I-axis and Q-axis, thereby resulting in dips as shown in FIG. 3. For example, when the phase rotation amount is 180 degrees, the amplitude dips to approximately zero because the phase change locus passes through the neighborhood of the zero point, and dips 3 dB when a 90 degree change occurs.
Thus, since the phase loci define a curve apart from the I-axis and Q-axis, finite times are required for the phase changes, and the level of the envelope changes at phase transition points.
The level changes which occur at these phase transitional points (1) are not involved in information transmission, and (2) widen transmission bandwidth due to the level changes, and are considered as drawbacks.
MSK as shown in FIG. 4 is known as another technique similar to PSK. The modulated signal by MSK is expressed by the following equation. EQU S(t)=[i.sub.k cos.omega..sub.r t]cos.omega..sub.c t+[q.sub.k sin.omega..sub.r t]sin.omega..sub.c t (2)
This shows that MSK is equivalent to using half-period sinusoidal waves, whose phases differ by 90 degrees from each other, instead of square pulses of the input signal used in the QPSK modulation circuit shown in FIG. 1. MSK is characterized in that it has little degradation in characteristics such as phase distortion even in a nonlinear transmission line having a saturation characteristic, because MSK includes no discontinuous transitions of the carrier waves and has an envelope of a constant amplitude.
The phase of MSK defines a spatial locus that rotates clockwise or counterclockwise in accordance with "0" or "1" in the transmission data. In other words, MSK transmits information using positive and negative rotational directions. MSK, however, does not use the non-rotational state as information.